1. Field of the Invention
The invention relates to inductive couplers for non-contacting power transmission, in particular to inductive rotating transmission devices and here particularly to inductive rotating transmission devices for computer tomographs. With these, the electrical power substantially needed for driving an X-ray tube is transmitted from a stationary side to a rotating side of a gantry of a computer tomograph. The transmission is here effected without contact by means of an inductive rotary joint that is built up similarly to a transformer in which the primary side and the secondary side are adapted to be rotated with respect to each other.
2. Description of Relevant Art
With units that are rotatable relative to each other, such as radar installations or also computer tomographs, in the same way as with linearly movable units such as crane systems or conveyor vehicles, it is frequently necessary to transmit electrical energy between movable units. In order to transmit this energy without contact, inductive couplers are preferably employed. These have the advantage over mechanical slide tracks or also slip rings that abrasion, wear, mechanical force outlay for moving the coupler, and also outlay of maintenance are substantially less. The term “inductive coupler” here relates to a circuit for generating an alternating current, together with an inductive transformer or rotary joint for energy transmission between two parts that are movable and, in particular, rotatable relative to each other.
Inductive rotating transmission devices as disclosed in U.S. Pat. No. 7,197,113, for example, have magnetic cores of iron or ferrite material and at least one winding on each side of the units that are rotatable relative to each other. An alternating current is fed into a first winding and tapped off via a second winding that is movable relative to the first one.
U.S. Pat. No. 7,054,411 shows a complete circuit of an inductive power transmission system for computer tomographs, together with the associated power electronics.
In DE Publication No. 102007006394, an inductive rotating transmission device is disclosed in which the output voltage is regulated by a control unit which, for this purpose, determines an electrical parameter on the primary winding of the rotating transmission device.
With conductively coupled slip rings it is simple to transmit a predetermined voltage from the stator side to the rotor side. Here only the relatively low ohmic losses need be taken account of. With inductive rotating transmission devices the stray inductance of the rotating transmission device plays a major part. It represents a frequency-dependent impedance that substantially affects the transmission characteristics of the rotating transmission device. This stray inductance depends upon various factors such as the inductance of the windings of the stator side and the rotor side, and also upon the magnetic configuration. Now, in order to transmit electrical energy via a rotating transmission device of this kind, a series capacity is connected in series for compensation. With this, a series resonance circuit results. At its resonance frequency this has an impedance of zero and here makes possible a transmission of high power. For control of the power flux the operating frequency can be chosen to differ from the resonance frequency.
Instead of a series resonance circuit, a parallel resonance circuit also can be constructed by connecting a capacity in parallel. The characteristics described in the following similarly apply to a parallel resonance circuit. At its resonance frequency the resonance circuit has an impedance of almost zero and here makes possible the transmission of high power. By changing the impedance, which is effected by changing the switching frequency, the output voltage can be controlled.
If the arrangement is now operated at a switching frequency that is optimal for certain load conditions, then a conversion of the output voltage at the load results at a certain ratio to the input voltage from the line. In most cases the input voltage from the line is a well-filtered DC (direct-current) voltage that in most cases even originates from a power-factor correcting circuit. However, in some applications a power-factor correcting circuit of this kind is not needed and can be dispensed with. Thus, only a rectified AC (alternating current) from the power line having a relatively high residual ripple is available as an input voltage of the rotating transmission device, which again results in a corresponding residual ripple of the output voltage at the load. In order to reduce the ripple of the voltage at the load, for example the output voltage can be measured and this measured value fed back to the stationary side. With conventional (not movable) power supply units a separate feedback transformer is used for this. However, with rotating transmission devices an additional rotating transmission device is needed, which causes additional cost and requires space.
Alternatively, the filter capacitors on the input side could be enlarged. However, this leads to a large volume of the arrangement and higher cost. Furthermore, the primary-side peak-current consumption from the line increases, whereby the higher harmonics in the line are increased.
Another possibility is to insert an additional converter stage on the rotating side between the secondary side of the rotating transformer and the output. Buck or boost converters are frequently used for these converter stages, but other converters such as Zeta or Ćuk converters are also possible. The input voltage of this downstream converter stage may fluctuate within a wide range with the output voltage being kept constant. However, this solution requires an additional converter on the rotating side, which increases the cost and also the weight, as well as the volume of the arrangement.